ABSTRACT Several interventions for mental health problems are efficacious and effective, but few are routinely offered to college students, who represent 59% of young adults. This is regrettable because college students are at high risk for mental health problems (e.g., depression, substance abuse, eating disorders), and college counseling centers lack sufficient clinicians to offer individual therapy to all afflicted students and are not well positioned to deliver prevention programs. One solution for this service shortfall is to have peer educators deliver scripted group-based prevention programs, which can more efficiently reduce the burden of mental illness than individual therapy. Targeting college students is a cost-effective tactic for delivering prevention programs and has vast potential reach because 85% of colleges have peer educator programs. Peer educators have effectively delivered several prevention programs, sometimes producing larger effects than clinicians, as was the case in a preliminary trial of a group-based prevention program with a particularly strong evidence-base. Guided by Wandersman et al. (2012), we propose to evaluate 3 levels of implementation support (training, technical assistance, and quality assurance/improvement) for the delivery of a prevention program. We will randomize 45 colleges to: (1) a Training condition where experts provide an intensive discrete 2-day initial train-the-trainer workshop that simultaneously trains peer educators to deliver the intervention and campus supervisors to train and support future peer educators, plus the facilitator guide and facilitator support website; (2) a Training + Technical Assistance condition, adding a day implementation training to articulate goals, needs, leadership structure, adoption options, recruitment strategies, and communication; or (3) a Training + Technical Assistance + Quality Assurance/Quality Improvement condition adding 1 year of technical assistance, coaching, and quality assurance to enhance implementation skills and sustainability. We will test whether greater implementation support is associated with graded increases in fidelity and competence in delivering the scripted prevention program (Aim 1), student attendance of the intervention and effectiveness of the program on pre-to-post changes in outcomes compared to usual care data collected before implementation (Aim 2), and reach and sustainability of the program (Aim 3). We will test whether Consolidated Framework for Implementation Research (CFIR) indices of perceived intervention characteristics, outer and inner setting factors, peer educator attributes, and process factors after the initial training correlate with fidelity, competence, attendance, effectiveness, reach, and sustainability over the implementation period and test whether at the end of the initial 1-year implementation period the 3 conditions differ on relevant CFIR indices and on the progress and timing of implementation (Aim 4). We will evaluate the prevention program delivery cost in the 3 conditions and the relative cost-effectiveness of each condition in terms of attaining intervention fidelity, competence, attendance, effectiveness, reach, and sustainability, as well as general cost-savings at the clinics (Aim 5).